Hello there @Markusz88 & @lemmie88!
I'm sorry to hear that you've encountered some trouble when trying to disable cloud saves. I've taken a closer look at this, and it looks like cloud saves cannot be permanently disabled without playing offline. This is due to the cross-progression support for Assassin's Creed: Valhalla. When you create a manual save, a copy of your save should be saved to your local disc as well as being uploaded to the cloud. This means that in the case of a degradation or outage of Ubisoft services, you can continue playing with your local save.
To my knowledge, the only way to truly prevent your save from being uploaded to the cloud would be to play offline, as an online connection is required for a save to be uploaded.
If you are concerned about losing progress due to corrupted cloud saves, I would recommend that you make regular manual saves, as these will be saved to your local disk. You can also then back these up manually.
If you have any further questions or concerns, please don't hesitate to reach out.
Thanks!